This invention relates to electrocoating surfaces of interior cavities of articles such as containers for foods and beverages, particularly metal cans. In electrocoating, an electric circuit is established between the surface being coated and the coating source so that the coating is ionically attracted to the surface. When the electrocoating process involves spraying the coating, it is important that the sprayed stream provide a continuous conductive connection between the spray nozzle and the surface being coated.
It has been known to electrocoat the interiors of containers by means of a spray nozzle which is inserted into the cavity. Examples of this approach may be seen in U.S. Pat. Nos. 3,922,213; 4,119,522; 4,094,760; 4,107,016; 4,210,507; 4,246,088; 4,400,251; 4,436,594; 4,515,677; and 4,883,578. With a nozzle inserted into the cavity, particularly if the nozzle body fills a substantial portion of the space in the cavity, maintaining continuity of liquid contact can be relatively easy. However, a drawback to that approach is that inserting and withdrawing the nozzle from each container limits the speed with which the containers can be processed. Nevertheless, that approach had been commonly proposed as the preferred method for electrocoating containers such as cans.
Avoiding inserting spray nozzles into each container has the potential for increased production line speeds, and such an arrangement is shown in U.S. Pat. No. 5,164,056. In that arrangement, the containers, with their open ends oriented generally downwardly, are supported on an open grate while the coating is sprayed upwardly into the interior cavities of the container from nozzles located within or below the grate. While this arrangement shows considerable potential for improved electrocoating line speeds, maintaining continuity of contact between the coating liquid and the interior surface of the container during electrocoating can be difficult since the spray nozzles are distant from the farthest extremity of the interior surface and there is considerable open space within the cavity. If flow of the liquid coating does not uniformly cover the surface to be coated during the electrocoating stage, the resulting coating layer will be of non-uniform thickness and in some areas may be of inadequate thickness.
In U.S. Pat. No. 5,164,056 it is suggested that uniform coverage of the interior of the container may be aided by using spiral shapes within the spray nozzles and by aligning the nozzles so that more than one nozzle directs coating into the interior at any given time. Further improvements would be desirable. It may also be possible to improve the filling of the interior by using high flow rates, but this approach can be uneconomical due to the large pumping requirements that are entailed. High pumping rates can also lead to the detrimental side effect of foaming. Coating flow pressures that are unduly high can be a problem in the arrangement of U.S. Pat. No. 5,164,056 in that tile containers can be lifted momentarily from the support means unless held down to close tolerances, which if not carefully maintained can result in containers being damaged by the conveying means. Metal cans that are not strengthened at the neck (such as by a narrowed neck portion) are particularly susceptible to being damaged in this way. In other arrangements, lifting of the container may break electrical contact which is required for electrocoating.
The problem of providing complete contact of the interior of the container with liquid coating material is particularly acute with metal cans that are straight-walled, i.e., not having a narrowed neck portion.